At present, natural biomaterials are predominantly used in reconstructive surgery, mainly in the fields of gastroenterology, urology and wound-healing, but with increasing applications in cardiovascular and cosmetic surgery. One such biomaterial, porcine small intestinal submucosa (SIS), has been used in gastroenterology, urology and wound-healing applications as it is easily incorporated into host tissue and remodelled. However, recent work to investigate the adherence and viability of human cells seeded onto SIS demonstrated that commercially-available SIS specimens contained porcine nuclear residues and was cytotoxic in vitro. In addition, clinical use of SIS has resulted in localised inflammation, suggesting the material can cause an immunological response in vivo. An alternative natural biomaterial is decellularised porcine dermis (Permacol™) which has a wide range of uses in medical and cosmetic procedures and has been implanted in over 8500 patients in over 70 different surgical procedures since being licensed for use in humans in 1998. However, Permacol™ a disadvantage associated with this material is that not only is it only partially resorbable but when it is used in an animal model of bladder augmentation it has been shown to cause micro-calcification and irregular detrusor regeneration. Furthermore, Permacol™ is unable to support in vitro recellularisation thus preventing its use as a biomaterial that could be seeded with cells and functionalised prior to implantation. An improved natural biomaterial that is immunologically inert and able to support recellularisation would offer an immediate advantage in the art.
It is estimated that over 400 million people worldwide suffer from some form of bladder dysfunction. A variety of diverse congenital and acquired conditions result in bladder dysfunction, for example cancer, congenital abnormalities, nerve damage or trauma. Currently, the major surgical solution is surgical reconstruction. It is known in the prior art to repair or augment or replace the bladder during these procedures with vascularised segments of the patient's own tissue derived from their stomach or more commonly their intestine. However, this latter procedure (‘enterocystoplasty’) is associated with significant clinical complications that arise due to the exposure of the epithelial lining of the intestine to urine. It has been found that the use of intestine results in significant complications, such as infection and development of bladder stones, as the intestine is lined by an absorptive and mucus-secreting epithelium that is incompatible with long-term exposure to urine. Consequently, a number of alternative approaches have been proposed to find a practical and functional substitute for native bladder tissue. One of the alternative solutions is ‘composite enterocystoplasty’, where the de-epithelialized intestine wall is lined with bladder epithelial cells that have been propagated in vitro, to augmenting the urinary system with natural or synthetic biomaterials that may incorporate in vitro-propagated cells. However even this modified form of enterocystoplasty has been associated with adverse side effects. The lack of an entirely satisfactory clinical procedure has led researchers to investigate alternative strategies.
Attempts have been made to develop suitable biomaterials for urological tissue engineering using both synthetic materials, for example polyglycolic acid and poly L-lactic acid, and naturally derived materials including SIS, Permacol™ and porcine bladder matrix. However none of these materials have been found to be totally successful vis a vis immunogenicity and rejection and recellularisation.
Tissue matrices prior to implantation undergo a process of decellularisation in order to reduce their immunogenicity once implanted. This process involves removing the donor cells, whilst ideally retaining the biomechanical structure and function of the matrix. As regards the bladder, it is especially desirable to maintain the normal mechanical properties and its elasticity.
A problem associated with the use of a bladder matrix is that bladder tissue is relatively thick (1-5 mm when not distended). This in turn means that it is difficult to decellularise bladder tissue to provide an immunologically inert scaffold matrix using routine methods known in the art. Attempts to decellularise dissected segments of full thickness porcine bladder have resulted in incomplete decellularisation. Histological analysis of these samples indicated that cells had not been removed from the muscular bladder wall. In its retrieved form, the porcine bladder was too thick to allow efficient penetration of the solutions used throughout the decellularisation process. In order to overcome this problem segments of bladder tissue of reduced thickness could be used to enable successful decellularisation.
An improved method of decellularisation that could be used to decellularise whole bladders or other membranous sacs of full thickness, whilst retaining the biomechanical properties of the tissue, would offer immediate advantage in the field of particularly but not exclusively, urological tissue engineering.